In devices having a laser source and heterodyne coherent detection, it may be necessary to determine a frequency change (Doppler effect) occurring over part of the path of the laser beam and resulting from a physical phenomenon that it is desired to compensate for or to measure. This is in particular the case with lidars used for measuring wind speed and direction by the backscattering of the laser beam from aerosol particles carried by the wind. This frequency change is generally determined by mixing the signal received with a signal generated by a local oscillator affected by a frequency shift that is produced by an electrooptic modulator or an acoustooptic modulator (AOM).
A device of this kind is also used to measure the speed of aircraft relative to the surrounding medium. In this case, the device is called an anemometer.
The measurement distance defines the type of detection of the Doppler shift to be measured and the power of the light source of the anemometer. Thus, the detection may either be coherent detection or direct or incoherent detection.
In the case of heterodyne coherent detection, the beam coming from the light radiation source (a laser) is split into two, one part being spatially shaped and sent to the measurement zone. An acoustooptic modulator shifts the frequency of the beam for the reference channel. Next, the backscattered signal is mixed with the shifted reference in order to generate interference in a detector.
In the anemometer application, a laser beam, generated by a laser source, is focused at a certain distance from the aircraft. Aerosols present in the atmosphere backscatter the incident beam, producing a shift in its emission frequency. The Doppler frequency, that is to say the difference between the frequency of the backscattering beam and the incident beam is detected by an interferometer in order to deduce the speed of the aircraft. It is known that the value of the Doppler frequency Fd is given by:Fd=2v/λ,v being the projection, onto the line of sight of the laser, of the aircraft speed relative to the ambient medium (the atmosphere), i.e. the reference with respect to which it is desired to measure the speed of movement of the aircraft, and λ being the wavelength of the emitted beam.
FIG. 1 shows a block diagram of an optical Doppler measurement device of the prior art, which uses a frequency shift device of the AOM type.
The device shown in FIG. 1 comprises in particular a laser unit 10, a mixing/detection unit 12 and an optical head 14, these components 10, 12, 14 corresponding to the main functions of the measurement device.
The laser unit 10 comprises a laser source SL and a polarization-maintaining coupler (PMC) 18 delivering a first optical signal for injection into a signal channel 20 and a second optical signal for injection into a reference channel 22.
The signal channel into which the first optical signal is injected comprises a fiber acoustooptic modulator (AOM) 24 shifting the frequency of the optical signal. The signal output by the AOM is preamplified and then amplified, by an optical preamplifier (Pamp) 26 and an optical amplifier (Amp) 28, respectively, which deliver an optical power signal to be emitted into the reference medium.
The power signal output by the signal channel 20 is injected, through a polarization-splitting coupler (PSC) 32 and an optical bidirectional link 34 of the mixing/detection unit 12, the optical head 14 radiating a laser beam Fem into the reference medium.
The optical head firstly focuses the emitted laser beam Fem in the reference medium and, secondly, detects the rays Frd backscattered by the medium in a specified direction.
The backscattered rays Frd detected by the optical head, which possibly include a Doppler shift, are sent via the bidirectional optical link 34 to the polarization-splitting coupler 32, which delivers, owing to the rotation of the polarization of the backscattered signal relative to the emitted signal, by means of a λ/4 optical plate 35, a backscattered optical signal Pr to a signal return output Sr.
The mixing/detection unit 12 further includes a polarization-maintaining coupler (PMC) 40 which receives, at one of its inputs, the reference signal POL output by the reference channel of the laser unit 10 and receiving, at another input, the backscattered signal Pr. The PMC coupler 40 mixes the two frequency-shifted optical signals injected into its two inputs, generating interference signals injected into a detector Dt 42.
Signal processing applied to the detector then allows extraction of the Doppler shift, measurement of the speed of movement v and the direction of movement.
Detection of the Doppler shift in this structure shown in FIG. 1 is coherent, of the heterodyne type (frequency shift by the AOM).
In this type of structure for measuring a Doppler shift, the use of an acoustooptic modulator AOM to shift the frequency of the reference channel beam has the drawback of generating harmonics liable to disturb the signal processing and, consequently, parasitic signals that limit the spectral measurement window and precision.